[form] Reimplement the "login" user interface

[form] Reimplement the "login" user interface

Rewrite the code implementing the "login" user interface to use a
predefined interactive form.  The command "login" then becomes roughly
equivalent to:

  #!ipxe

  form
  item          username   Username
  item --secret password   Password
  present

with the result that login form customisations (e.g. to add a Windows
domain name) may be implemented within the scripting language.

Signed-off-by: Michael Brown <mcb30@ipxe.org>